DC/AC converter with improved starter circuit

Abstract

A circuit arrangement for operating a discharge lamp comprises a self-oscillating DC/AC converter (II) provided with a series arrangement of a first and a second switching element between a first and a second input terminal (2, 2') for connection to a DC voltage source (III). The series arrangement supplies an alternating current Ib to a load branch having output terminals for connection of the discharge lamp during nominal operation. The circuit arrangement is in addition provided with a starter circuit ST for generating a DC voltage component between the control electrode (1c') and the main electrode (1a') of the second switching element (1'). The starter circuit also include apparatus which comprise an auxiliary switch (25) that brings the first switching element (1) into a conducting state if the amplitude of the alternating current Ib is low compared with its amplitude during nominal operation. This apparatus prevents damage to the switching elements and promotes the initiation of the nominal operational state.

We claim: 
     
       1. A circuit arrangement for operating a discharge lamp comprising: a self-oscillating DC/AC converter (II) including: a series arrangement of a first switching element and a second switching element connected between a first input terminal and a second input terminal for connection to a DC voltage source, wherein said series arrangement, during nominal operation, supplies an alternating current (Ib) to a load branch which comprises at least first capacitive means, inductive means, and output terminals for connection of the discharge lamp,   the load branch having a first end connected to a junction point situated in the series arrangement between the first and the second switching elements, and a second end connected to one of the input terminals,   wherein the switching elements each has a control electrode and a main electrode to which a control circuit is connected having means for the generation from the alternating current (Ib) of a control signal for the first and the second switching elements respectively,     a starter circuit comprising means for generating a DC voltage component between the control electrode and the main electrode of the second switching element, and wherein the starter circuit further comprises auxiliary means for placing the first switching element in a conducting state at an amplitude of the alternating current (Ib) which is low with respect to its amplitude during nominal operation, said auxiliary means comprising an auxiliary switching element.   
     
     
       2. A circuit arrangement as claimed in claim 1, characterized in that the auxiliary switching element is controlled by a further control signal originating from the control circuit of the first switching element. 
     
     
       3. A circuit arrangement as claimed in claim 1, characterized in that the auxiliary means further comprise a voltage divider which shunts the input terminals and which comprises a first branch and a second branch which have a common junction point connected to the control electrode of the first switching element, said first branch having a first partial branch which is connected to the second input terminal and a second partial branch which is connected to the common junction point of said branches, and the partial branches have a common junction point connected to the first input terminal via the auxiliary switching element. 
     
     
       4. A circuit arrangement as claimed in claim 3, characterized in that the auxiliary switching element is shunted by a reverse oriented unidirectional element connected in a branch between the first and the second input terminal. 
     
     
       5. A circuit arrangement as claimed in claim 1, characterized in that the means for generating the control signal from the alternating current (Ib) comprise a secondary winding of a transformer which has a primary winding in the load branch. 
     
     
       6. A circuit arrangement as claimed in claim 1 wherein, during a start-up phase of the DC/AC converter, said auxiliary switching element is controlled into a cut-off state as long as the alternating current (Ib) is below said low amplitude, whereby both of said first and second switching elements conduct current simultaneously. 
     
     
       7. A circuit arrangement as claimed in claim 6 wherein said auxiliary switching element is controlled into a conducting state as long as the alternating current (Ib) is at or above said low amplitude, whereby said first and second switching elements receive the control signal that alternately switches them on and off. 
     
     
       8. A circuit arrangement as claimed in claim 1 wherein the load branch further comprises second capacitive means in shunt with the output connection terminals for the discharge lamp, said second capacitive means and said inductive means forming a resonant circuit. 
     
     
       9. A circuit arrangement as claimed in claim 1 further comprising a second capacitive means coupled to the control electrode and main electrode of the first switching element, and said means for the generation of the control signal comprises a transformer having a primary winding connected in the load branch in series with the inductive means and a secondary winding coupled to the control electrode and main electrode of the first switching element, wherein the second capacitive means and said secondary winding form a resonant circuit. 
     
     
       10. A circuit arrangement as claimed in claim 9 further comprising voltage-breakdown means connected to the control electrode and said main electrode of the first switching element. 
     
     
       11. A circuit arrangement as claimed in claim 1 wherein said starter circuit includes means for applying a further DC voltage to the control electrode of the first switching element during a start-up phase when the amplitude of the alternating current (Ib) is below said low amplitude, and said auxiliary switching element is controlled into a cut-off state as long as the alternating current (Ib) is below said low amplitude, and is coupled to the control electrode and said main electrode of the first switching element so that, when the auxiliary switching element is conducting, it effectively removes the further DC voltage from the control electrode of the first switching element. 
     
     
       12. A circuit arrangement as claimed in claim 1 wherein said auxiliary switching element is coupled to the control electrodes of the first and second switching elements and is controlled into a conducting state as long as the alternating current (Ib) is at or above said low amplitude, whereby said auxiliary switching element remains conductive while the first and second switching elements receive the control signal that alternately switches them on and off. 
     
     
       13. A circuit arrangement as claimed in claim 1 wherein said auxiliary switching element is coupled to the control electrodes of the first and second switching elements, and further comprising a diode connected in anti-parallel with the auxiliary switching element. 
     
     
       14. A circuit arrangement as claimed in claim 1 wherein said auxiliary switching element is coupled to the control electrodes of the first and second switching elements and is controlled by the control signal determined by the alternating current (Ib) in the load branch.